Mode interferometer squinting radar antenna

ABSTRACT

Lateral displacement of the phase center of the electromatic wave feeding a reflector from a feed horn is achieved by interference between a plurality of modes created by combining suitably adjusted and mixed waves from each of two waveguides into a single waveguide which is twice the width of either. The displaced phase center provides a squint angle, off of boresight, to the wave reflected from the reflector.

United States Patent 1 [75] Inventor: Irving 1. Goldmacher, Stamford,

' Conn.

g Primary ExaminerEli Lieberman -1 Assigneei 3"" S g Corporation, East Attorney-Melvin Pearson Williams art or onn. 22 Filed: Jan. 21, 1972 57] ABSTRACT [2!] Appl. No.: 219,731

' Lateral displacement of the phase center of the electromatic wave feeding a reflector from a feed horn is 343/781 3 3 322 achieved by interference between a plurality of modes Fie'ld 778q781 created by combining suitably adjusted and mixed i waves from each of two waveguides into a single waveguide which is twice the width of either. The displaced I v phase center provides a squint angle, off of boresight, ['56] NifE g g z z sE s giiE -N rs to the wave reflected from the reflector. 2,994,869 8/1961 Woodyard; 343/777 10 Claims, 8 Drawing Figures 90 CMQPzfA/f Goldmacher 1 June 19, 1973 i [54] MODE INTERFEROMETER SQUINTING 3,530,483 9/l970 Pierrot 343/786 RADAR ANTENNA 3,423,756 1/1969 Foldes 343/786 2,981,946 4/1961 Korman 343/781 PATENTEU 3.740.752 v JOMQCf mam/r I MODE INTERFEROMETER SQUINTING RADAR ANTENNA BACKGROUND OF THE INVENTION 1. Field of Invention This invention relates to radar antennas, and more particularly to improvements in antenna feed relating to squinting of the beam patternoff of the reflector boresight.

Description of the Prior Art The scanning or movement of the primary lobe of radar beams has long been known in the art. An initial method of scanning a radar beam includes orienting the antenna, including the feed horn and reflector, in the desired primary beam direction. However, since the mechanical structure of radar antennas is frequently very complex and must operate in confined areas (particularly in airborne installations) a recent addition to the radar art is the phased array radar which, without a reflector, steers a beam in a desired direction by means of a single wave created by the interference of a plurality of individual waves of mutually different phases. However, such antennas are known tob e quite costly because of the large number of relatively expensive phase shifters required toimp lement the antenna.

' As acompromise between the purely mechanical and purely wave-generating scanning of a radar beam,

radar antennas have been provided with a squint? function which is the ability'to orient the beam in a direction slightly off the boresight of the reflector. This has been achieved mechanically, either by adjusting the lateral position of the feedhorn with respect to the reflector, or by adjusting the lateral position of the reflecthereover render the mechanical squinting antenna unsuitable in many applications.

Attempts have been made to utilize two'side-by-side .feedhorns and varying the phase of them so as to direct the wave toward the reflector in a different direction in .the fashion of a phase interferometer, thereby to alter the point at which a wavefront will intersect with the parabolic reflector. But this has not provided the desired result because the incident wavefront has to be propagating in a direction which is parallel with the boresight of thereflector, whereas a phase interferometer will rather project the wavefront at'an angle to the boresight.- 1

SUMMARY OF THE INVENTION A The primary object of the present invention is to provide non-mechanical squinting to a beam of electromagnetic energy emanating from a reflector.

According to the present invention, a complex waveguide assembly includes two channels with means disposed in at least one of them to provide a relative phase shift between the waves in'each of them, means for coupling a portion of the relatively phase adjusted wave in each channel into the other channel with an additional phase adjustment, the last named means feeding a combining waveguide section having a primary dimension substantially twice that of either" of the wave- 2 guide sections of the two channels, thereby to produce at least two modes of the original wave which interfere in a manner to provide a singlecombined wave, the effective phase center of which can be located laterally across the last section by adjusting the original mutual phase difference between the waves. In further accord with the present invention, phase adjusted coupling means may comprise a short slot cross coupler providing coupling of substantially 50 percent of each wave into the other channel'and a 90 phase shift. In still further accord with the present invention, the mixing sec tion includes means to provide. electrical tuning thereof, thereby to support a less dominant mode therein.

The invention provides a relatively simple and low cost means of achieving beam squinting. It may be electronically adjusted at high speeds, and can be extremely accurately controlled.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawing's.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-4 are illustrations of transverse electromagnetic waves relevant to the present invention; FIGS. 5-7 are schematicized illustrations of the effects of lateral displacement ofan approximate phase ce'nterof a wave illuminating a reflector; and- 7 FIG. 8 is a simplified, partially broken away perspective view of one embodiment of the present invention.

DESCRIPTION OF THE PREFERREDv EMBODIMENT Consider, as an introduction to a detailed description of a preferred embodiment of the invention, the analysis of electromagnetic waves in a waveguide, modes thereof, and the manner in which the present invention manipulates the modes to achieve the objects hereof.

Referring now to FIG. 1, an' end view ofa waveguide 10 has super posed thereon amplitude vectors 12 illustrating an electromagnetic wavein a TE mode. In FIG. 2, the end view of the waveguide 10 has superposed thereon amplitude vectors l4 illustrative of an electromagnetic wave in the TE mode. As illustrated in FIG. 3, with the two modes (TE TE linearly superposed on one another (that iswith the electricfields coaligned) in an interfering relationship (such as within the same waveguide), the amplitudes of the waves add vectorially so as to produce a single combined wave as illustrated by the dashed line 16. The illustration in FIG. 3 presupposes that the two waves are in phase with each other, they differing only by the mode (that is one half wave between waveguide walls or two half waves between waveguide walls). In FIG. 3, the TE mode is shown as being so related to the TE mode that the left half of the TE mode is in phase with the TE mode. By adjusting the relative phase (which would appear in a direction orthogonal to the coordinates of FIG. 3) by varying amounts, the. relationship between the two modes can be adjusted. By adjusting the phase of the TE mode from that illustrated in FIG 3, the relationship of FIG. 4 results. The significant point is that, by keeping the amplitudes of both modes on a one to one basis and by relatively adjusting their phases, the configuration of the total wave shape can be shifted so as to achieve a total wave having a different mode configuration as illustrated by the dashed line 18 in FIG. 4. For the two modes shown, the position of the maxima of the waves 16 and 18 represent the maximum displacements to the right and the left (as in FIGS. 3 and 4). Adjustments in phase between and 180 Of the T mode (relative to those shown in FIGS. 3-and 4) would provide total waves having amplitude maxima inbetween those of the two waves 16, 18. In other words, adjusting the phase of the TE mode relative to the TB mode can adjust the lateral position of the maxima of the total wave. This maxima can be considered, in terms of a'feedhorn feeding a reflector in a radar antenna, as a phase center of the wave emanating from the feedhorn toward the reflector, in an approximate sense. That is to say, one can consider the phase-center to be the point of maximum amplitude of a transverse wave emanating from a feedhorn toward a reflector. 7

According to a known basic law of physics, changing of the phase center of the energy illuminating a parabolic'reflector produces a phase change of the energy in the aperture plane of the antenna, which changes the direction of the secondary pattern (the reflected energy) as it leaves the reflector. The present invention is predicated onutilizin'g the shifting of the approximate phase center in the illumination of a reflector so as to alter the direction of the primary wavefront from the reflector, thereby to provide mode interferometer control over the direction of the wavefront. Thus, as is illustrated in FIG. 5, if a feedhorn 20 is located at the focal point of a parabolic reflector 22 and irradiates the reflector 22 with a TE wave 24 having its maxima, and therefore the phase center of the wave, located atthe center ofthe feedhorn 20, then the secondary wave reflected from, the reflector 22 will be collimated and oriented coaxially with the boresight of the reflector 22 as illustrated by the arrows 26.

In contrast, FIG. 6 illustrates that with the phase center of thewave 28 emanating from the feedhorn shifted laterally (in a manner similar to that illustrated in FIG. 3), then the approximate phase center of the wavefront is similarly shifted (upward in FIG. 6). This produces waves, the spherical wavefronts of which emanating toward the reflector have centers on a locus moved upward from theboresight of the reflector 22 as seen in FIG. 6, producing a'ph'ase shift in differential portions of the wavefront as it crosses the aperture plane of the reflector 22 so that the secondary wave reflected from the reflector 22 emanate at an angle with respect to the boresight of the reflector 22, as illustrated by the arrows 30. Similarly, FIG 7 illustrates that shifting of the maximum amplitude of the wavefront emanating from the feedhorn 20 (as illustrated by the dashed line 32) downwardly as seen in FIG. 7, causes'an opposite affect to'that illustrated inFIG. 6 so that the reflected wave is reflected at an angle whichis upward (as seen in FIG. 7) from. the boresight of the reflector 22, as illustrated by the arrows 34.

In accordance with the present invention, the configuration of the wavefront emanating from a waveguide or a feedhorn may be altered in a number of ways. One preferred embodiment of apparatus suitable for producing the desired mode interference in accordance with the present invention is illustrated in FIG. 8.

Therein the feedhorn 20 is seen to comprise the final section (designated E in FIG. 8) of a complex waveguide assembly 21.

In FIG. 8, radiation enters the complex waveguide assembly 21 through openings 33, 34 at a proximal end 35 thereof, which comprises two separate waveguides along section A of the complex waveguide assembly. For purposes herein, it is assumed that both of the waves are of the type illustrated in FIG. 1 (TE,,, mode) and in phase with each other. These waves may be derived from a single waveguide in a given radar system by using a short slot coupler (of the type well known in the art) with a suitable phase shift, or by using a Y split-v ter; or by use of any other known meansto achieve two separate waves from one.

Thw two separate waveguides continue into section B of the complex waveguide assembly. However, in one side, a phase shifter 44 is disposed. This may suitably comprise a ferrite section 46 having an electrical winding 48 therethrough and a pair of matching sections 50, 52 disposed on opposite ends of the ferrite 46. Such a structure is disclosed in a copending application of the same assignee entitled A TEMPERATURE STABI- LIZED LATCI-IING PHASE SHIFTER, Ser. No. 185,974, filed on Oct. 4, 1971 by Peter W. Smith. In the event that the present invention is to be practiced in an antenna system which is to be reciprocal, and therefore useful both in transmit and receive modes, the direction of current flow through the winding 48 in thetransmit mode would be reserved from that in the receive mode. On the other hand, any other suitable phase shifter, of a reciprocal type, (or of a nonreciprocal type with a suitable switching provision made therefor) may be utilized as desired to suit any implementation of the present invention. Control over the amount of phase shift within the phase shifter 46 is dependent upon the magnitude of average current through the device, which can be controlled with a DC current intensity or with pulsewidth modulation, in any known fashion that is found suitable. The current winding 48 may be suitably controlled by a squint control circuit 54, which may comprise a simple, manually ad justed current control (such as a potentiometer) or it may comprise a more complex control mechanism such as a well known beam steering computer suitably adv justed to control current.

Within the section C of'the complex waveguide of FIG. 8, the function of a short-slot coupler is provided due to the fact that a pair of metallic walls 56, 58am separated, or spaced apart, so that radiation in each waveguide spills into the other, on a substantially fifty fifty basis, with a phase lag in the transmitted portion of the wave. That is, a wave entering through the opening 33 will provide 50 percent of its power into a channel 59 of the waveguide; and that portion of the wave transmitted into the waveguide channel will have a 90 phase lag with respect to the original wave that passes through the phase shifter 46, and with respect to the other half of the wave in a channel 60 of the waveguide. The wave entering through opening 34 is similarly split. The next section, D, again has the waves separated into two waveguide sections, one of which includes a second phase shifter 61 in which the electrical winding 62 is controlled in a constant fashion by a voltage source 64 so as to provide a constant phase shift through the phase shifter 60 of substantially 90.

The last section, E, is a single waveguide, having twice the dimension of the individual waveguide sections (33,

60; 34, 59), within which the waves in the two waveguide sections can interfere with one another. The two waves that enter the final section, E, are TE mode waves each having an amplitude and a phase which is dependent upon the variable phase shift provided-by the phase shifter 46 and the fixed phase shift provided by the phase shifter 60, the losses in these phase shifters, the fact that 50 percent of the power of one is coupled into the other in section C, and so forth. These waves are of varying amplitude and phase (both absolutely and with respect to each other) as a function of the control provided by the squint control 54 to the phase shifter 46. Ideally, relative phase shifts between the two channels of O to 180 are desired; this can be achieved by adjusting the phase shifter 46 commensurately between 0 and 180.

As the two TE modes enter section E from the respective waveguide channels, they interfere with each other so as to produce a single TE mode across the entire dimension of section E and a TE mode across the entire section which is equivalent to two TE modes as they enter at 180 phase shift to each other, as illustrated in' FIGS. 3 and 4 and described hereinbefore. These waves add together and result in a total wave (which is of the type described and illustrated by dashed lines in FIGS. 3-7 hereinbefore) emanating through the open, distal end63 of the waveguide.

The invention may be more simply derived, if desired, by using separate waveguides for sections A, B, D and E with a known short slot coupler inserted inbetween the separate waveguides as a substitute for the section C.

Although the TE mode has no trouble existing in section E of the complex waveguide assembly, the TE 7 mode in section E can be sufficiently perturbed by the wall 58 so as to attenuate it significantly unless tuning is provided. For this purpose, a tuning screw 68 may be adjustably inserted through the bottom (or top) of the waveguide. As is known in the art, a variable amount of insertion will tune section E to promote the existence of the TE mode. The screw 68 may be located along the central axis somewhere midway between the end of the wall 58 and the opening 62.

The phase shifter 46 is preferably located very near to the right end of the separating wall 56. On the other hand, however, the phase shifter 60 should be located some distance from the right-hand end of the separation wall 58 so that the high dielectric constant of the phase shifter 60, making the waveguide appear to be wider than it really is, will not allow penetration of high order modes into that section of the waveguide. A free length of waveguide to the right (as seen in FIG. 8) of the phase shifter 60, with air dielectric therein, provides an electrical width which is too short to support other than the TB", mode of'the individual waveguide.

It may be found to be advantageous, because of the heavy loading that the losses in the phase shifters 46, 61 provide to one channel to provide similar losses in the other channel. Thus dielectric may be inserted in the unshifted channel (34, 59) simply to provide a loss match between the two channels, or, the significant thing is only the relative phase shifts and amplitudes between the two waves is significant (and not their absolute values), phase shifters with a fixed phase shift and dielectric loss may be put into the channel (34, 59) for operation differentially with the phase shifters 46, 60.

tain the 90 phase shift and substantially In a similar fashion, phase shifter 46 may be left in the position shown, and the phase shifter 61 may be put in the channel 59, instead of as shown. The only difference then would be a different phase required within the phase shifter 46 which is accommodated by the particular current provided by the squint control 54.

The cross coupler (section C) preferably provides a substantially 50 percent split of the energy in each channel (causing a coupling of half of the energy of each into the other). Otherwise, there is a tendency for the amplitude of the TE mode in section E to not be equal to the amplitude of the TE section mode in section E. Despite the phase shifter 61 being set for a phase shift of 90 on the other hand, if other than substantially a 50 percent coupling is provided in section C, this can be accommodated by adjusting, on a dynamic and continuous basis the phase provided in the phase shifter 61. Since this is a complex function to perform and may be very costly, it is preferable to repercent inter-channel coupling in section C. In the case where the cross coupling provided in section C is ideal (within a fractional percent of 50 percent coupling into opposing waveguides) then a very simple device, such as a dielectric insert which automatically provides a 90 phase shift, could be substituted for the phase shifter 61 and the current source 64. i

The length of the void between the walls 56, 58 is on the order of half a waveguide wavelength (as is known, for a given frequency, the wavelength is longer in the waveguide than it is in free space, and as used herein wavelength refers to the waveguide wavelength).

Since the nature of the phase shifter 61 is dependent upon the precise apparatus used to provide cross coupling between the two channels, it may be considered together with cross coupling means as a means for providing cross coupling at a given phase shift; in that context, the areas and 71 on opposite sides of the wall 58 which extend beyond any phase shifter 61 may be considered to be a pair of waveguides fed by the cross coupling and phase adjusting means.

Although the invention has been shown and described with respect to preferred embodimentsthereof, it should be understood by those skilled in the art that the foregoing and various other changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Having thus described typical embodiments of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for coupling electromagnetic waves between waveguides and a reflector, comprising:

a first section comprising a first pair of waveguides and means for providing a variable relative phase .shift between waves in'said waveguides, said waveguides each being of substantially the same primary dimension;

a second section comprising a second pair of waveguides having the same primary dimensions as said first pair of waveguides and means for providing a relative phase shift to wave energy in said second pair of waveguides;

a short slot coupler intercoupling said first section with said second section; and

a fifth waveguide having a primary dimension substantially twice that of said first and second waveguides, said fifth waveguide connected to said second pair of waveguides.

2. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 1, further comprising:

tuning means in said fifth waveguide adapted for adjustment to electrically tune said fifth waveguide to support a desired wave therein.

3. Apparatus for coupling electromagnetic waves be tween waveguides and a reflector according to claim 1 wherein said short slot coupler provides coupling of substantiallySO percent of the wave energy in each one of said first pair of waveguides with each one of said second pair of waveguides.

4. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 3 wherein said phase shift means of said second section comprises means for providing a substantially 90 phase shift.

5. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 4 wherein said variable relative phase shift means is adjustable to provide relative phase shifts of approximately to approximately 180.

6. Apparatus for coupling electromagnetic waves between waveguides and a reflector, comprising:

a first section comprising first and second waveguides and means for providing a variable relative'phase shift between waves in said waveguides, said waveguides beingof a primary dimension to support a TB mode of a wavetherein;

a second section comprising third and fourth waveguides having substantially the same primary dimension as said first and second waveguides;

coupling means for=coupling and phase shifting wave energy between said first section and said second section, said coupling means coupling a substantial portion of wave energy in said sections between said first and fourth waveguides with a substantial phase shift, coupling a substantial portion of wave energy in said sections between said second and third waveguides with a substantial phase shift, coupling a substantial portion of wave energy in said sections between said first and third waveguides .with a minor phase shift and coupling a substantill portion of wave energy between said second and fourth waveguides with a minor phase shift, said coupling means providingan additional relative phase shift between wave energy in said third waveguide and wave energy in said fourth wave guide; and

stantially twice that of said first and second waveguides, said fifth waveguide connected to said second pair of waveguides.

7. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 4, further comprising:

tuning means in said fifth waveguide adapted for adjustment to electrically tune said fifth waveguide to support a desired wave therein.

8. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 6 wherein said coupling means provides coupling of substantially 50 percent of the wave energy in each one of said first and second waveguides with each one of said third and fourth waveguides.

9. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 8 wherein said additional relative phase shiftmeans comprises means for providing a substantially phase shift.

10. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 9 wherein said variablerelative phase-shift means is adjustable to provide relative phase shifts of approximately 0 to approximately fifth waveguide having a primary dimension sub- 

1. Apparatus for coupling electromagnetic waves between waveguides and a reflector, comprising: a first section comprising a first pair of waveguides and means for providing a variable relative phase shift between waves in said waveguides, said waveguides each being of substantially the same primary dimension; a second section comprising a second pair of waveguides having the same primary dimensions as said first pair of waveguides and means for providing a relative phase shift to wave energy in said second pair of waveguides; a short slot coupler intercoupling said first section with said second section; and a fifth waveguide having a primary dimension substantially twice that of said first and second waveguides, said fifth waveguide connected to said second pair of waveguides.
 2. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 1, further comprising: tuning means in said fifth waveguide adapted for adjustment to electrically tune said fifth waveguide to support a desired wave therein.
 3. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 1 wherein said short slot coupler provides coupling of substantially 50 percent of the wave energy in each one of said first pair of waveguides with each one of said second pair of waveguides.
 4. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 3 wherein said phase shift means of said second section comprises means for providing a substantially 90* phase shift.
 5. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 4 wherein said variable relative phase shift means is adjustable to provide relative phase shifts of approximately 0* to approximately 180*.
 6. Apparatus for coupling electromagnetic waves between waveguides and a reflector, comprising: a first section comprising first and second waveguides and means for providing a variable relative phase shift between waves in said waveguides, said waveguides being of a primary dimension to support a TE10 mode of a wave therein; a second section comprising third and fourth waveguides having substantially the same primary dimension as said first and second waveguides; coupling means for coupling and phase shifting wave energy between said first section and said second section, said coupling means coupling a substantial portion of wave energy in said sections between said first and fourth waveguides with a substantial phase shift, coupling a substantial portion of wave energy in said sections between said second and third waveguides with a substantial phase shift, coupling a substantial portion of wave energy in said sections between said first and third waveguides with a minor phase shift and coupling a substantill portion oF wave energy between said second and fourth waveguides with a minor phase shift, said coupling means providing an additional relative phase shift between wave energy in said third waveguide and wave energy in said fourth waveguide; and a fifth waveguide having a primary dimension substantially twice that of said first and second waveguides, said fifth waveguide connected to said second pair of waveguides.
 7. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 4, further comprising: tuning means in said fifth waveguide adapted for adjustment to electrically tune said fifth waveguide to support a desired wave therein.
 8. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 6 wherein said coupling means provides coupling of substantially 50 percent of the wave energy in each one of said first and second waveguides with each one of said third and fourth waveguides.
 9. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 8 wherein said additional relative phase shift means comprises means for providing a substantially 90* phase shift.
 10. Apparatus for coupling electromagnetic waves between waveguides and a reflector according to claim 9 wherein said variable relative phase shift means is adjustable to provide relative phase shifts of approximately 0* to approximately 180*. 